Conventional centrifugal pumps usually comprise, as shown in FIG. 7, a housing 300 inside which driving magnetic means 230 is circumferentially disposed around a rotation axis (not explicitly designated in the drawings). The housing 300 is secured to a motor 250 (only a portion thereof is shown in FIG. 7). The driving magnetic means 230 is secured to a spindle of the motor 250 and supported thereby so as to be rotatable about the rotation axis with the spindle of the motor 250. The housing 300 has an opening to receive therein a rear cover 220 to seal the housing 300.
The rear cover 220 has a central recess which is generally concentric with the driving magnetic means 230 and receiving therein driven magnetic means 224 which is circumferentially disposed around the rotation axis and is concentric with the driving magnetic means 230 so that when the driving means 230 is rotated by the motor, the driven means 224 follows the driving means 230 due to the magnetic force therebetween. To bearingly support the rotation of the driven means 224, a fixed central shaft 221 with a bushing 222 encompassing therearound is concentrically disposed inside the driven means 224. Retainers 226 are also disposed on the fixed central shaft 221 to keep the bushing 222 in position.
A front cover 210 overlaps the rear cover 220 and secured thereto in such a way that an interior is formed therebetween to receive therein an impeller 225. The impeller 225 has an extension toward the central recess of the rear cover 220 to cover the driven means 224, forming a plastic enclosure 223 thereof, so that when the driven means 224 rotates about the fixed central shaft 221, the impeller 225 follows the rotation thereof. The front cover 210 also forms a spiral configuration for discharging the pumped fluid with a discharging port 212 on a lateral location thereof. The front cover 210 also has a suction eye 211 on a central and front portion thereof to draw in fluid to be pumped.
Friction between the bushing 222 and the fixed shaft 221 results in heat generated therebetween during rotation. A fluid passage 240 is formed along the outside surface of the plastic enclosure 223 with a first end thereof communicating the fluid discharging port 212 and a second end thereof communicating a plurality of spaced cooling grooves 255 which are helically or circumferentially formed on the inside surface of the bushing 222, i.e. the surface in contact with the fixed central shaft 221 so as to conduct the pumped fluid therethrough along the arrows shown in FIG. 7 to the cooling grooves 255. A returning passage 260 in communication with the cooling grooves 255 conducts the fluid back to the impeller 225.
With the circulation of fluid within the fluid passages 240 and 260 and the cooling grooves 250, the heat generated between the bushing 222 and the fixed central shaft 221 is brought away and thus the bushing is prevented from being overheated.
However, when the operation of the centrifugal pump is abnormal, such as unloaded operations caused by, for example, control device malfunction, inadequate operation, block-up of ducts, insufficient fluid level, the operation usually results in a significant increase of temperature in both the bushing 222 and the fixed shaft 221. Further, the high temperature deforms the plastic enclosure 223 of the driven means 224 so as to cause wear and abrasion of the plastic enclosure 223 and thus damage to the pump.
To overcome the deformation of the enclosure 223 resulted from a high temperature, pieces of material (not shown) which are able to bear high temperatures are attached to the enclosure 223. This, however, is not very effective, because a long period of unloaded operation of a centrifugal pump usually results in a temperature over 220 degrees Celsius and using heat-resistance materials is not sufficient to protect the enclosure. Besides, adding the heat-resistance material also increases the difficulty and cost of manufacture.
It is therefore desirable to provide a centrifugal pump of which the unloaded operation will not cause a significant temperature increase inside the pump for a very long period, as compared with the conventional centrifugal pump structures.